The following description will be made by taking a piezoelectric oscillator as an example of such a semiconductor device formed by stacking two packages. Due to rapid progress in price-reduction and miniaturization of mobile communication apparatuses, such as a mobile phone, along with the popularization thereof, miniaturization and thinning are also increasingly expected in piezoelectric oscillators, such as a crystal oscillator, for use in the communication apparatuses
Heretofore, as a structure of a piezoelectric oscillator, there has been known a type which comprises a first package embedding a circuit element for making up an oscillator circuit, and a second package stacked on and fixed to the first package while internally housing a piezoelectric vibrator element, as disclosed in the following Patent Document 1. In the Patent Document 1, it is intended to achieve miniaturization of a piezoelectric oscillator in such a manner that a lead frame for electrically connecting the first package and a mounting board to which the piezoelectric oscillator is mounted, and a lead frame for electrically connecting the first frame and the second frame, are formed as separate lead frames, and the separate lead frames are arranged to allow respective positions thereof in a vertical direction to overlap each other.
However, along with the popularization of mobile phones, the needs for miniaturization and thinning of communication apparatuses and electronic apparatuses are increasingly growing, and thereby the technique of simply stacking two lead frames as disclosed in the Patent Document 1 can no longer respond to the needs. Specifically, in the technique of the Patent Document 1, due to the vertically overlapped arrangement, the lead frames are located closely to cause difficulty in smooth wiring, so that there is a limit to narrowing a pitch between the lead frames for miniaturization and thinning. Moreover, if a thickness of the lead frame is reduced with a view to miniaturization and thinning, strength of the lead frame will be lowered, which causes problems, such as deformation of the lead frame due to handling during assembling, or impossibility of handling in some cases.
On the other hand, the applicant of this application disclosed a package structure of a piezoelectric oscillator, using a transfer lead frame, in the following Patent Document 2. In the package structure, a metallization wiring is formed by resin-molding a package using a transfer lead frame having the metallization wiring arranged on a base substrate, and, after the resin molding, remove the base substrate of the transfer lead frame to leave only the metallization wiring on a removing surface of a molded resin in a transferred manner, while allowing a peripheral region of the metallization wiring to be exposed on the side of in flush relation with the removing surface of the molded resin so as to serve as an external terminal. This makes it possible to further reduce a thickness to facilitate miniaturization and thinning, as compared with the structure where a plurality of lead frames is arranged in vertically overlapped relation in a single package.
However, the package structure using the transfer lead frame also has limitations in miniaturization and thinning, particularly, a reduction in planar size (length×width). Specifically, an attempt to reduce a planar size of a package causes a need for avoiding short-circuiting between a circuit element and an external terminal of a metallization wiring. Thus, there is no other choice but to reduce a planar size of the external terminal of the metallization wiring so as to prevent interference therebetween. However, if the planar size of the external terminal of the metallization wiring is excessively reduced, it becomes impossible to obtain a desired mounting strength. Therefore, due to a restriction for ensuring the desired mounting strength, there is a limit to a reduction in planar size of a package.
[Patent Document 1] JP 2004-166230A
[Patent Document 2] JP 3947545B